Our long-term aim is to understand how the ultraviolet radiation in sunlight causes skin cancer. In previous grant periods we progressed from biophysical events to the tissue level, linking DNA photoproducts to sunlight-induced mutations in the p53 and PTCH tumor suppressor genes, and p53 to UV-induced apoptosis and UV-driven clonal expansion ofp53-mutant keratinocytes. Now, we focus more closely on a novel aspect of the molecular mechanism of UV-induced apoptosis. We recently found that p53;E2fl double knockout mice regain their proficiency for UV-induced apoptosis (as well as their cancer resistance), in striking contrast to the defect in p53 knockouts. We hypothesize that: a) P53 is not an essential trigger of apoptosis, but instead regulates a "direct UV-apoptosis pathway" that is triggered by its own UV induction signal, b) P53 upregulates this pathway by opposing basal suppression by B2fl. The p53; E2fl double knockout is thus a unique reagent for studying the direct UV apoptosis pathway without complications from simultaneous UV effects on its regulatory apparatus. The present Aims seek to: i) identify the direct UV-apoptosis pathway; ii) characterize its P53-E2fl regulator; and iii) characterize the signal that triggers UV-apoptosis in the direct pathway. These experiments will clarify the molecular pathway through which UV radiation -- and, by inference, sunlight -- induces apoptosis in skin cells. Dividing the system into two parts each needing a separate UV signal, a "direct" operating part and its P53-E2fl regulatory apparatus, may also resolve the relative contributions of damage to DNA, RNA, and membrane receptors in UV signaling. At a clinical level, understanding the E2fl mechanism may lay the basis for circumventing p53-related resistance to cancer therapy.